The present invention relates generally to the field of marine geophysical surveying. More particularly, in one or more embodiments, the present invention relates to methods and systems for towing one or more electromagnetic sensor streamers from deflectors in an additional geophysical sensor streamer spread.
Certain types of marine geophysical surveying, such as seismic or electromagnetic surveying, include towing an energy source at a selected depth in a body of water. Actuation of the energy source emits an energy field into the body of water. The energy field interacts with the rock formations below the water bottom with changes in the energy field due to this interaction detected by geophysical sensors positioned in the water. The detected energy is used to infer certain properties of the subsurface rock, such a structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
Current electromagnetic survey techniques are generally based on towing an electromagnetic source in the body of water while a number of electromagnetic sensors are disposed at a substantially stationary position at the water bottom. Electromagnetic survey systems have also been developed that include towing both the electromagnetic source and a sensor streamer having electromagnetic sensors disposed thereon at spaced apart locations. As the streamer is pulled through the water, one or more hydrodynamic depressors can be used to pull the streamer down to a pre-selected depth. The length of the lead-in cable interconnecting the streamer with the survey vessel can be adjusted to regulate depth of the streamer in combination with the depressors. More fine depth adjustments can be made with depth control devices cooperatively engaged with the streamer. In some instances, it may be desirable to tow more than one sensor streamer in parallel in an electromagnetic survey. To achieve the desired separation between the sensor streamers (e.g., 500 meters), specially designed submersible deflectors have been proposed for providing the lateral component of force needed to create the desired lateral spread between streamers while also providing downward force to maintain the streamers at the desired towing depth. However, the development and operation of the submersible deflectors may add undesired cost and complexity to the electromagnetic survey system.
Accordingly, there is a need for improved methods and systems for marine electromagnetic surveying.